Isolation and Phenotypic Characterization of Culturable Bacteria Associated with Spoiled Onion Bulbs (Allium cepa L.) Sold in Selected Markets of Dutse Metropolis, Jigawa State, Nigeria

Spoilage of onion bulbs causes economic losses and may pose health risks, and bacterial contamination is one of its main contributors. This study isolated and characterised culturable bacteria associated with spoiled onion bulbs sold in Dutse metropolis, Jigawa State, Nigeria. Ten spoiled onion samples (five each from Hakimi Street and Sabuwar Kasuwa markets) were aseptically prepared, serially diluted, and plated on nutrient agar. Isolates were characterised by colony morphology, Gram staining, microscopy, and biochemical tests. Colony counts ranged from 5.5 × 10⁴ CFU/mL (H5) to 4 × 10⁵ CFU/mL (H4) among Hakimi Street samples and from 5.5 × 10⁴ CFU/mL (SW3) to 4.5 × 10⁵ CFU/mL (SW4) among Sabuwar Kasuwa samples. Phenotypic characteristics indicated Staphylococcus spp. (30%), Micrococcus spp. (30%), Escherichia coli (20%), and Klebsiella spp. (20%). All isolates were catalase positive; the Staphylococcus isolates were coagulase positive, whereas E. coli, Micrococcus spp., and Klebsiella spp. were coagulase negative. Because identification relied on phenotypic characteristics only, the isolates are presumptive, and their occurrence indicates association with spoiled onions rather than proof that they caused the spoilage. Further studies using molecular identification and pathogenicity tests are recommended to establish species-level identities and the organisms responsible for spoilage.